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Japan's Kanto region, which includes the city of Tokyo on the
main island of Honshu, is one of the most seismically active
areas on Earth. Situated near the triple junction of the Pacific,
Philippine and Eurasian plates, the Kanto region lies along the
famed Pacific Ring of Fire and has experienced more than its fair
share of earthquakes and tsunamis.

Historical records document dozens of quakes in the Kanto region
over the last millennium, but it's been difficult for scientists
and historians to figure out how often the largest of those
quakes, dubbed the great Kanto earthquakes, recur.

Now, a team of Japanese scientists has combined those historical
records with geochemical and depositional analyses of the
region's soil to better pinpoint the Kanto region's
major historical quakes.

"The interval of the great Kanto earthquakes has variation," said
Haengyoong Kim, a geophysicist at the University of Tokyo who
co-authored the study. "The recurrence interval is not at all
even," he said, making it difficult for scientists to know when
to expect the next major quake.

Shells, gravel and tsunamis

The most recent great Kanto earthquake struck Honshu in 1923. The
magnitude-7.9 quake killed more than 105,000 people — nearly
three of every 200 Tokyo residents at the time — lifted the Miura
Peninsula more than 5 feet (1.5 meters), and triggered a tsunami
that was at least 16 feet (5 m) high, which itself claimed
another 300 lives. [ 10 Biggest
Earthquakes in History ]

About 200 years earlier, in 1703, a magnitude-8.2 Kanto quake
caused 10,000 casualties. This quake also lifted the Miura
Peninsula and caused a 16-foot (5-m) tsunami, according to
historical reports.

But the region's written records become a little spotty before
the 1703 quake, so Kim and his colleagues turned to
geological evidence to figure out when earlier great Kanto
earthquakes struck the region.

Kim's team examined sediment cores from tidal flats on the Miura
Peninsula. In them they found three distinct bands of shells and
gravel — evidence of three large tsunamis, they say.

"Such abrupt changes of sedimentary environment were caused by
coseismic crustal movements," Kim and his team wrote in their
study. In other words, the only way to explain the dramatic
shell-gravel layers is through
tsunamis triggered by major earthquakes.

The team then used radiometric dating to deduce the ages of the
three layers. They found that the top band of gravel was
deposited in the early- to mid-1900s, and the second was
deposited after 1650, correlating well with records of the 1923
and 1703 earthquakes.

The third layer was deposited between the years 1060 and 1400,
they found. To pinpoint the specific date, the team went back to
the Kanto region's trove of written records. They were able to
match the oldest tsunami layer to a major earthquake in 1293 that
killed more than 23,000 people and severely damaged cities and
temples.

Unpredictable intervals

Overall, Kim and his colleagues found that the time between
Kanto's large temblors varies widely, from about 200 to 400
years.

Although the 200-year interval separating the two most recent
quakes was much shorter than the 400-year interval between the
first two, scientists
can't predict that the next quake will come any sooner, Kim
said.

"Our dataset does not indicate — and this study does not mean —
that the time interval to the
next future earthquake will be short," Kim told
OurAmazingPlanet.

"The magnitudes and source areas vary in each of the past Kanto
earthquakes," Kim said, meaning that the next quake may be just
as complex — and just as difficult to predict — as the first
three great Kanto earthquakes.

The team's findings were published Dec. 30 in the Journal of
Geophysical Research-Solid Earth.